Method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article

ABSTRACT

A method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising heating a mixture of boric acid or a borate and a chromium halide to its fusing state and immersing the article in the treating molten bath of said mixture, thereby forming a very hard chromium carbide layer on the surface of said article. The method of this invention can be carried out without ageing the treating bath and can form a very smooth chromium carbide layer on the surface of the article.

United States Patent [191 Komatsu et al.

[ 51 May 20, 1975 [75] Inventors: Noboru Komatsu, Toyoakeshi;

Tohru Arai, Nagoyashi; Yoshihiko Sugimoto, Nagoyashi; Masayoshi Mizutani, Nagoyashi, all of Japan [73] Assignee: Kabushiki Kaisha Toyota Chuo Kenkyusho, Nagoyashi, Japan [22] Filed: Apr. 11, 1974 [21] Appl. No.: 460,149

[30] Foreign Application Priority Data Apr. 12, 1973 Japan 48-40823 June 22, 1973 Japan 48-69752 [52] U.S. Cl 427/228; 427/399 [51] Int. Cl B44d l/06 [58] Field of Search ..117/114 R, 113,169 R,

[56] References Cited UNITED STATES PATENTS 2,694,647 11/1954 Cole 117/130 R 3,671,297 6/1972 Komatsu et a1 117/118 Primary Examiner-Mayer Weinblatt Assistant ExaminerEdith L. Rollins Attorney, Agent, or Firm-Wenderoth Lind & Ponack [57] ABSTRACT A method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising heating a mixture of boric acid or a borate and a chromium halide to its fusing state and immersing the article in the treating molten bath of said mixture, thereby forming a very hard chromium carbide layer on the surface of said article. The method of this invention can be carried out without ageing the treating bath and can form a very smooth chromium carbide layer on the surface of the article.

9 Claims, 3 Drawing Figures METHOD FOR FORMING A CHROMIUM CARBIDE LAYER ON THE SURFACE OF AN IRON, FERROUS ALLOY OR CEMENTED CARBIDE ARTICLE This invention relates to a method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article, and more particularly it relates to the formation of the carbide layer on the surface of the article immersed in a treating molten bath. The iron. ferrous alloy or cemented carbide article with the carbide layer formed thereon has a greatly improved hardness, wear resistance, oxidation resistance and corrosion resistance.

There have been reported several kinds of methods for coating or forming a metallic carbide layer on the surface of metallic articles. We have developed a method for forming a chromium carbide layer on the surface of metallic article in a treating molten bath consisting of boric acid or a borate and a metal powder containing chromium (US. Pat. No. 3,671,297). The method can form a uniform carbide layer and is highly productive and cheap. The chromium carbide (Cr C Cr C has a very high hardness ranging from Hv 1,800 to Hv 2,000. Therefore, the carbide layer formed represents a high value of hardness and a superior resistance performance against wear and is thus highly suitable for the surface treatment of moulds such as dies and punches, tools such as pinchers and screwdrivers, parts for many kinds of tooling machines, automobile parts to be subjected to wear.

Further the chromium carbide represents a greater resistance against oxidation and corrosion than the tungsten carbide forming cemented carbide does.

Therefore, the formation of the chromium carbide layer on the surface of cemented carbide increases greatly the durability of the mold, jig and the like used in the corrosion atmosphere and in high temperature.

The conventional method mentioned above requires a relatively long time for preparing the treating bath due to slowness of the dissolution of the treating metal particle and sometimes the treating metal particles happens to deposite into the carbide layer formed and to make the surface of the layer rough.

Therefore, it is the principal object of this invention to provide an improved method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath.

It is another object of this invention to provide a method for forming a chromium carbide layer with denseness and uniformity and without any undissolved treating metal particles on the surface of the article.

It is still another object of this invention to provide a method for forming a chromium carbide layer, which is safe and simple in practice and less expensive,

It is a still further object of this invention to provide a treating molten bath which does not necessiate aging of the bath and is capable of forming a carbide layer having a smooth surface on the iron, ferrous alloy or cemented carbide article.

Other objects of this invention will appear hereinafter.

The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention, itself, as to its method of operation, together with additional objects and ad- '2 vantages therefore, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings. in

which: i

FIG. 1 is a photomicrograph showing a chromium carbide layer formed on the surface of a carbon tool steel accordingto Example I;

FIG. 2 is a photomicrograph showing a chromium carbide layer formed on the surface of a carbon tool steel according to Example 2;

FIG. 3 is a photomicrograph showing a chromium carbide layer formed on a cemented carbide according to Example 3.

Broadly, the present invention is directed to an improvement of the method for forming a chromium carbide layer on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath and is characterized in that the treating molten bath is composed of boric acid or a borate and a chromium halide and in that the iron, ferrous alloy or cemented carbide article to be treated contains at least 0.05% by weight means by weight hereinafter) of carbon, Namely, the method of the present invention comprises preparing a treating molten bath consisting of boric acid or a borate and a chromium halide and immersing the iron, ferrous alloy or cemented carbide article into the treating molten bath so as to form the chromium carbide layer on the surface of said article.

In the method of this invention, a chromium halide is employed as a main ingredient of the treating molten bath instead of the metallic powders containing chromium used in the previously developed method mentioned above. Said halide can easily dissolve in a molten boric acid or borate and does not remain as solid particles. Therefore, the treating molten bath can be used as soon as the treating material is molted without the aging of the bath and a very smooth carbide layer can be formed on the surface of the article.

In order to prepare the treating molten bath, a boric acid or a borate and a chromium halide are mixed together and then the mixture is heated to its fusing state, or the boric acid or borate is heated to their fusing state and then the halide is added into the molten boric acid or borate. As the chromium halide, chromium chloride (CrCl CrClchromium fluoride (CrF chromium bromide (CrBrchromium iodide (Crl and the like can be used. As the borate, sodium borate (borax) (Na B O potassium borate (K B,O and the like can be used. In the treating molten bath, one or more than one kind of the halide and a borate, boric acid or the mixture of boric acid and a borate can be used. The boric acid and borate have a function to dissolve a metallic oxide and to keep the surface of an article to be treated clean, and also the boric acid and borate are not poisonous and hardly vaporize. Therefore, the method of thepresent invention can be carried out in the open air.

The chromium halide may be included in the treating molten bath in a quantity between about I to 50%. With use of less quantity of the chloride than 1%, the formation of the carbide layer would not be uniform and would be too slow to be accepted for the practical purpose. The addition of the halide more than 50%, the viscosity of the molten bath becomes too high to be normally operated and the corrosiveness of the treating molten bath becomes too strong. The most preferable quantity of the halide may be 5% or thereabout.

The remainder of the treating molten bath is boric acid, borate or the mixture thereof. Said boric acid or borate may be mixed in a quantity between 50 and 99%. In order to lower so called melting point and the viscosity of the treating molten bath, salt such as sodium chloride. potassium chloride and sodium fluoride. oxide such as phosphoric oxide, hydroxide such as sodium hydroxide, sulphate. carbonate. nitride and the like can be added into the treating molten bath.

The iron, ferrous alloy or cemented carbide article to be treated must contain at least 0.057! of carbon, and should preferably contain 0.1% of carbon or higher. The carbon in the article becomes to be a composition of the carbide during the treatment. Namely it is supposed that the carbon in the article diffuses to the surface thereof and reacts with the chromium from the treating molten bath to form the carbide on the surface of the article. The higher content of the carbon in the article-is more preferable for forming the carbide layer. The iron, ferrous alloy or cemented carbide article containing less than 0.05% of carbon may not be formed with a uniform and thick carbide layer by the treatment. Also, the article containing at least 0.05% of carbon only in the surface portion thereof can be treated to form a carbide layer on the surface of the article. For example, a pure iron article, which is case-hardened to increase the carbon content in the surface portion thereof, can be used as the article of the present invention.

Here, iron means iron containing carbon and casehardened iron, ferrous alloy means carbon steel and alloy steel, and cemented carbide means a sintered tungsten carbide containing cobalt. Said cemented carbide may include a small amount of titanium carbide, niobium carbide, tantalum carbide and the like.

In some cases, the carbon contained in the treating molten bath can be used as the source of the carbon for forming the carbide layer on the surface of the article. However, the formation of the carbide layer is not stable and the use of the carbon in the treating molten bath is not practical.

Before the treatment, it is important to purify the surface of the article for forming a good carbide layer by washing out the rust and oil from the surface of the article with acidic aqueous solution or another liquid.

The treating temperature may be selected within the wide range from the melting point of boric acid or borate to the melting point of the article to be treated. Preferably, the treating temperature may be selected within the range from 700 to l,lC. With lowering of the treating temperature, the viscosity of the treating molten bath increases gradually and the thickness of the carbide layer formed decreases. However, at a relatively high treating temperature, the treating molten bath is worsened rapidly. Also the quality of the material forming the article is worsened by increasing the crystal grain sizes of said material.

One of the merits of the present invention is to enable the treatment at a relatively low temperature such as 700C. As well known, when an article is treated at a temperature above the transformation temperature of the material forming the article, the article would be deformed due to its hidden stress. However, most of the articles can be treated without being deformed by selecting a treating temperature which is lower than the transforming point of the material forming the article and above the lowest treating temperature 700C. In

the case that the material forming an article to be treated has a relatively low transformation point, the treating temperature can be lowered by adding the above mentioned compounds for lowering the viscosity of the treating molten bath into the treating molten bath and the article may be treated at a temperature below its transformation point.

The treating time depends upon .the thickness of the carbide layer to be formed. Heating shorter than l0 minutes will, however, provide no practically accepted formation of said layer. although the final determination of the treating time depends on the treating temperature. With the increase of the treating time, the thickness of the carbide layer will be increased correspondingly. In practice, an acceptable thickness of the layer can be realized within 30 hours or shorter time. The preferable range of the treating time will be from 10 minutes to 30 hours.

The vessel for keeping the treating molten bath of the present invention can be made of graphite or heat resistant steel.

It is not necessary to carry out the method of the present invention in the atmosphere of non-oxidation gas, but the method can be carried out into effect either under the air atmosphere or the inert gas atmosphere.

EXAMPLE 1 500 grams of borax was introduced into a graphite crucible of 65mm inner diameter and heated up to 1,000C for melting the borax in an electric furnace under the air, and then 82 grams of chromium chloride (CrCl powder of under mesh was poured into the molten borax and mixed together for preparing a treating molten bath. Next, a specimen, 7mm diameter and 40mm long, made of carbon tool steel (.lIS 5K4, containing 1.0% of carbon) was immersed into the treating molten bath and kept therein for about 2 hours, taken out therefrom and air-cooled. Treating material adhered to the surface of the specimen was removed by washing with hot water and then the specimen treated was investigated. The surface of the specimen was very smooth. After cutting and polishing the specimen, the specimen was micrographically observed, and it was found that a layer shown in FIG. 1 was formed. The thickness of the layer was about 8 microns. And the layer was identified to be chromium carbide (Cr C Cr C by X-ray diffraction method and by an X-ray microanalyzer. Boron was not detected from the specimen treated.

EXAMPLE 2 500 grams of borax was introduced into a graphite crucible of 65mm inner diameter and heated up to l,000C for melting the borax in an electric furnace under the air, and then grams of chromium fluoride (CrF powder under 100 mesh was poured into the molten borax and mixed together for preparing a treating molten bath. Next, a specimen, 7mm diameter and 40mm long, made of carbon tool steel (JIS 5K4), was immersed into the treating molten bath and kept therein for about 3 hours, taken out therefrom and aircooled. Treating material adhered to the surface of the specimen was removed by washing with hot water. The surface of the specimen treated was very smooth. After cutting and polishing the specimen, the cross section of the specimen was micrographically observed and tested by X-ray diffraction method and by an X-ray microanalyzer, The layer formed was identified to be chromium carbide (Cr C Cr;C;,) and the thickness of the layer was about 12 microns. Said layer was shown in FIG. 2.

EXAMPLE 3 100 grams of borax powder was introduced into a graphite crucible and heated up to 1,000C for melting the borax in an electric furnace under the air, and then 46 grams of chromium chloride (CrCl powder of under 100 mesh was poured into the molten borax together with mixing up. Next. a specimen, lmm thick, 5.5mm wide and 30mm long made ofcemented carbide composed of 91% of tungsten carbide and 9% of cobalt was immersed into the treating molten bath and kept therein for hours, taken out therefrom and aircooled. Treating material adhered to the surface of the specimen was removed by dipping the specimen into hot water. The surface of the specimen treated was smooth. After cutting and polishing the specimen, the cross sectional area of the specimen was micrographically observed and tested by X-ray diffraction method and by X-ray microanalyzer. By the observation, a layer shown in FIG. 3 was found, and the thickness of the layer was 10a. By X-ray diffraction method, strong chromium carbide (Cr C diffraction lines were detected from the layer. By X-ray microanalyzer, the layer was found to contain a large amount of chromium. The hardness of the layer measured from the surface of the specimen was about Hv (Micro Vickers Hardness) 2,983. Also the hardness of the mother material of the specimen was measured to be about Hv 1,525.

Next, in order to evaluate the treatment, specimens treated by the same way mentioned above were subjected to either an oxidation test or corrosion test. As references. non-treated specimens were also tested. The oxidation test was to heat a specimen in open air at 800C for 1 hour and then to measure the weight gain of the specimen due to the oxidation of the specimen. The corrosion test was to dip a specimen into an aqueous solution of nitric acid (HNO for 50 hours and then to measure the weight loss of the specimen due to the dissolution of the specimen.

The oxidation gain of the treated specimen was 5.83 mg/cm In comparison, the oxidation gain of the non treated specimen was 61.87 mglcm Also, the dissolved weight loss of the treated specimen was 1.96 mg/cm In comparison, the weight loss of the non-treated specimen was 23.07 mg/cm It is apparent from the results that the cemented carbide having a chromium carbide layer thereon has a great oxidation and corrosion resistance.

EXAMPLE 4 In the same manner as described in Example 3, a treating molten bath composed of 100 grams of borax and 27 grams of chromium fluoride (CrF was prepared. Then a specimen having the same sizes and made of the same material as the specimen in Example 3 was treated for 13 hours at 1,000C. By the treatment. a layer of about 8p. thick was formed on the surface of the specimen. Also the layer was tested by X-ray diffraction method, X-ray microanalyzer and Vicker's Hardness Tester. And strong chromium carbide (Cr C diffraction lines were detected. The layer was found to contain a large amount of chromium.

Other specimens treated in this example were subjected to either the oxidation test or the corrosion test which were the same as those described in Example 3. The oxidation weight gain of the treated specimen was 6.15 mg/cm and the dissolved weight loss of the treated specimen was 3.25 mglcm What is claimed is:

l. A method for forming a carbide layer on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising the steps of preparing the treating molten bath consisting essentially of chromium halide and a member selected from the group consisting of boric acid. borate and the mixture thereof, immersing the article containing at least 0.05% of carbon into said treating molten bath. keeping said article in said treating molten bath for forming a chromium carbide layer on the surface of said article, and taking said article out of said treating molten bath.

2. A method according to claim 1, wherein said chromium halide is selected from the group consisting of chromium chloride, chromium fluoride, chromium bromide, chromium iodide and the mixture thereof.

3. A method according to claim 1, wherein said borate is sodium borate or potassium borate.

4. A method according to claim 1, wherein said article is immersed into the treating molten bath consisting essentially of 1 to 50% of said chromium halide and the balance of said boric acid or borate and keeping said article in said treating molten bath for 10 minutes to 30 hours at a temperature ranging from 700 to 1,100C.

5. A method according to claim 1, wherein said article is made of one selected from the group consisting of carbon steel and alloy steel containing at least 0.05% of carbon.

6. A method according to claim 1, wherein said cemented carbide article is made of tungsten carbide and cobalt.

7. A method according to claim 1, wherein the step of preparing the treating molten bath comprises heating boric acid or borate up to its fusing state, adding the chromium halide into said molten boric acid or borate mixing said chromium halide and said molten boric acid or borate.

8. A method according to claim 1, wherein the step of preparing the treating molten bath comprises preparing the mixture of the chromium halide and boric acid or borate and heating said mixture up to its fusing state.

9. A method according to claim 1, wherein said treating molten bath contains chloride or fluoride of an alkali metal for lowering the viscosity of the treating mol- 

1. A METHOD FOR FORMING A CARBIDE LAYER ON THE SURFACE OF AN IRON FERROUS ALLOY OR CEMENTED CARBIDED ARTICCLE IN A TREATING MOLTEN BATH, COMPRISING THE STEPS OF PREPARING THE TREATING MOLTEN BATH CONSISTING ESSENTIALLY OF CHROMIUM HALIDE AND A MEMBER SELECTED FROM THE GROUP CONSISTING OF BORIC ACID, BORATE AND THE MIXTURE THEREOF, IMMERSING THE ARTICLE CONTAINING AT LEAST 0.05% OF CARBON INTO SAID TREATING MOLTEN BATH, KEEPING SAID ARTICLE IN SAID TREATING MOLTEN BATH FOR FORMING A CHROMIUM CARBIDE LAYER ON THE SURFACE OF SAID ARTICLE, AND TAKING SAID ARTICLE OUT OF SAID TREATING MOLTEN BATH.
 2. A method according to claim 1, wherein said chromium halide is selected from the group consisting of chromium chloride, chromium fluoride, chromium bromide, chromium iodide and the mixture thereof.
 3. A method according to claim 1, wherein said borate is sodium borate or potassium borate.
 4. A method according to claim 1, wherein saiD article is immersed into the treating molten bath consisting essentially of 1 to 50% of said chromium halide and the balance of said boric acid or borate and keeping said article in said treating molten bath for 10 minutes to 30 hours at a temperature ranging from 700* to 1,100*C.
 5. A method according to claim 1, wherein said article is made of one selected from the group consisting of carbon steel and alloy steel containing at least 0.05% of carbon.
 6. A method according to claim 1, wherein said cemented carbide article is made of tungsten carbide and cobalt.
 7. A method according to claim 1, wherein the step of preparing the treating molten bath comprises heating boric acid or borate up to its fusing state, adding the chromium halide into said molten boric acid or borate mixing said chromium halide and said molten boric acid or borate.
 8. A method according to claim 1, wherein the step of preparing the treating molten bath comprises preparing the mixture of the chromium halide and boric acid or borate and heating said mixture up to its fusing state.
 9. A method according to claim 1, wherein said treating molten bath contains chloride or fluoride of an alkali metal for lowering the viscosity of the treating molten bath. 